This invention pertains to flywheel energy storage devices and more particularly to a flywheel device with an output regulator that produces a direct current output to a load, and maintains the direct current output voltage within an allowable range by switching the number of windings that couple to the load as the flywheel slows during a discharging. The invention provides for both higher efficiency, economy and reliability by eliminating the need for a high frequency output switching conversion.
Flywheels have emerged as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems, alternative energy generation, satellites and electric vehicles, Flywheel systems convert back and forth between electrical energy and the rotational energy of a spinning flywheel. A flywheel energy storage system includes a flywheel, a motor and generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor and generator converts between electrical and mechanical while the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low-pressure enclosure to minimize aerodynamic losses that would occur from operation in air at atmospheric pressure.
One typical requirement in the design of flywheel energy storage devices is to provide a near constant output voltage in order to power an electrical load as the flywheel speed slows during discharging. Unfortunately, as the speed of a flywheel slows, the voltage generated for a given generator field strength diminishes. For permanent magnet motor/generators, the field strength is constant so the voltage generated is directly proportional to the speed. Thus, if the flywheel slows to one quarter its full speed, the output voltage drops by a factor of four. Accordingly, the manufacturers of flywheel energy storage devices have used several methods for providing a near constant output voltage.
One such method has been to use an alternator type generator that uses a field coil for generation of the operating magnetic field. As the flywheel slows, the generator maintains a nearly constant output voltage by simply increasing the current to the field coil. This method is very simple, however it does have some drawbacks. The use of an electrically generated field requires a constant power draw and also potentially a magnetic circuit with higher magnetic losses from eddy currents and hysteresis depending on the design. These reduce the efficiency and require use of a larger flywheel if the expected discharge period is lengthy, up to several hours. Other potential drawbacks of alternator generators are significantly larger and heavier construction, smaller magnetic air gaps and generation of higher magnetic destabilizing forces that can make implementation of magnetic bearings, if used, more difficult.
Another prior art approach to the problem is the use of permanent magnet motor/generators with electronic switching conversion. Permanent magnet motor/generators, using permanent magnets to generate the magnetic field for operation, typically offer the highest efficiencies. Unfortunately, as previously explained, the output voltage from the generator falls as the flywheel speed slows. Electronic switching conversion can be used to provide a constant output voltage. One such prior art electronic switching conversion arrangement, shown in FIG. 1, is a power system 30 for providing back-up power to a protected load 29 from a flywheel energy storage device using an output DC-DC converter. The power system 30 has input power lines 31 energized from a source of power 39, such as the power grid, and output power lines 32 to the protected load 29. Input power in lines 31 is rectified by a rectifier 33 and provided to a DC buss 34. Power from the DC buss 34 is then provided to the output load 29 via output lines 32 through use of a DC-DC converter 38. Typical DC-DC converters chop the DC input 34 by switching, and put it back together as a regulated DC output power on lines 32. Switching of converters usually occurs at high frequencies, around 20 kHz.
In the event of loss of the primary input power from the source 39, back up power to the protected load 29, is provided by a flywheel motor/generator 37 driven by a flywheel in the flywheel energy storage device. A motor drive 35 connected to the DC buss 34 converts the DC to synchronous AC in lines 36 to energize the motor/generator 37 to accelerate the flywheel to its normal operating speed. When primary power in the line 31 is interrupted, the motor drive 35 instantly and automatically supplies power back to the DC buss 34 by rectifying the motor/generator AC power in lines 36. The power provided to the DC buss 34 falls as the flywheel speed is slowed. However, the DC-DC converter 38 converts the varying DC buss voltage 34 to a constant DC output in the lines 32. A special wide range DC-DC converter can be used to provide constant output voltage 32 during the entire useable flywheel discharge. Unfortunately, switching DC-DC converters typically have efficiencies that range from 75-90% efficiency. Even if the motor/generator has high efficiency, significant energy is lost in the output switching conversion to maintain the constant voltage. A second drawback of power systems with conventional converters is that the high frequency switching reduces the life of the electronics, which can limit the life of the flywheel energy storage device.
A second method for providing a constant output voltage while using a permanent magnet generator is to operate the motor drive in the fourth quadrant. A power system for a flywheel energy storage device using fourth quadrant power conversion of the motor drive inverter to provide output power is shown in FIG. 2. The power system 40 is comprised of a rectifier 43 that rectifies input power delivered from an input power source 41, such as a power grid, over lines 48, and supplies DC power to a DC buss 44, which is also the output to the load 42. Back up power is supplied through use of a flywheel motor/generator 47. A motor drive 45, connected to the DC buss 44, converts the DC power to synchronous AC to accelerate the flywheel motor/generator 47 to normal operating speed. During an interruption of primary power 41, the flywheel motor/generator 47 supplies the output power to the load 42 via lines 46 by reverse conversion from the motor drive 45. The motor drive 45 is a capable of fourth quadrant operation and hence it can actively slow the flywheel motor/generator 47 and in doing so, it can provide a constant and higher output voltage 42 than the back emf from the motor/generator. High frequency switching similar to that which is employed in the power system of FIG. 1 is used. Unfortunately, this power system 40 also suffers from similar power losses due to the high frequency switching and has the same life limitation considerations.
Thus, it would be very desirable to have a flywheel energy storage device with a power device that can employ a permanent magnet motor/generator and supply useable DC output power with high efficiency.
The invention provides a flywheel energy storage device with an output regulator that supplies direct current power to a load with high efficiency. The output voltage tolerated by many loads can be allowed to vary substantially, although not as much as would be encountered over an entire discharge from a flywheel with direct permanent magnet generator output. In telecommunications, one promising applications for flywheels and in many cases an application with a longer term discharge period, the voltage used by downstream equipment has an allowable range. Many DC telecommunications equipment for power of phone lines, wireless, Internet, etc., have embedded DC-DC converters. The DC-DC converters are provided so that the equipment can operate when batteries provide reserve power and the battery voltage falls during discharging, Most equipment has an allowable voltage range from either 40-60 volts or 20-30 volts. This substantial allowable operating voltage range, instead of requiring a specific voltage like 24 volts or 48 volts, allows an opportunity for designing a flywheel energy storage device to achieve a very high efficiency.
The output regulator in accordance with this invention maintains the output voltage within the allowable range by switching the number of windings or coil turns that couple to the load as the flywheel slows during a discharging. A permanent magnet generator attached to the flywheel produces AC voltage that varies in frequency and amplitude with the flywheel speed as it discharges. The output regulator switches to increase the number of windings that couple to the load during the discharge such that as the voltage falls to the lower end of the allowable range, whereupon it is instantly increased and then continues to drop again as the speed slows. The output regulator converts the generator AC to DC, and multiple turns of coils are combined electrically as the speed slows. The electrical coils switched are preferably located in the generator itself for simplicity and lower cost, however it is also possible to use an external transformer for switching of the magnetic coupling. No output DC-DC switching DC-DC converter is required and a high efficiency permanent magnet excited motor/generator can be used. Besides use for DC powered telecommunications equipment, the regulated DC output power can also power a standard inverter to power AC loads. Because most conventional inverters are designed to accommodate a voltage swing from batteries, the invention can be used to provide input power.
In one embodiment of the invention, the motor and generator are combined and the motor is accelerated to a higher voltage than the DC voltage required by output loads. The generator has multiple phases and each phase is separately rectified and smoothed to provide output power. Initially, output power is taken from only a single phase, however as the flywheel speed slows, the power from the other phases is added in series to maintain the output voltage within the allowable range. In other embodiments, multiple phases can be switched in parallel or power can be taken from only a single phase. The invention provides for both higher efficiency and reliability by eliminating the need for a high frequency output switching conversion while allowing use of a permanent magnet generator.